Television-receiving circuit arrangement



June 5, 1962 G. FCJRSTER ETAL TELEVISION-RECEIVING CIRCUIT ARRANGEMENTFiled June 16, 1959 L PRIOR ART INVENTORS GERHARD FORSTER OTTO HILKE BYusmur aAu ss W FIGA AGENT United States 3,037,565 TELEVISION-RECEIVINGCmCUIT ARRANGEMENT In contrast-control of television receivers, inparticular by variation of the screen-grid voltage of thevideoamplifying tube, a shift of the working point of the picture tube,which is conductively connected to the videoamplifying tube, is known tohappen so that the background brightness increases considerably whenincreasing the contrast.

it is already known to mitigate this inconvenience by applying thevideo-voltage to the cathode of the picture tube via a voltage dividerwhich is connected between the anode of the video-amplifying tube andthe bias-controlled screen-grid. As a result, the cathode of the picturetube is influenced by the greatly varying anode voltage and also by theoppositely varying screen-grid voltage. By a judicious choice of thevoltage divider it can be achieved that the voltage on the picture tube,which determines the black level, is independent of the set contrast.

However, the voltage divider connected in the videocircuit involves adisturbing increase in impedance of this circuit, hence a greaterinfluence of the stray capacitances which, in turn, adversely alfect thereproduction of the high video-frequencies which are of importance forgreater detail of the picture.

A circuit arrangement for contrast control is also known in which thescreen grid of the video-amplifying tube is connected to the cathode ofan auxiliary triode, the anode of which is connected to the positiveterminal of the supply through a resistor. To the control grid of theauxiliary tube there is applied a variable direct voltage which istransmitted to the screen grid of the video-amplifying tube. From theanode of the auxiliary tube a direct voltage is taken which variesoppositely with regard to the screen grid voltage of the videoamplifyingtube and by means of which the bias of the control grid of the picturetube is modified. Thus it is achieved that the anode-voltage variationof the videoamplifying tube, which variation is determined by thescreen-grid voltage variation and simultaneously occurs at the cathodeof the picture tube, is compensated by a corresponding variation of thevoltage on the Wehnelt cylinder of the picture tube, to the effect ofmaintaining the background brightness constant. However, such circuitarrangements are exceedingly complicated and have, in particular, thedisadvantage that the cathode of the auxiliary tube has a high directvoltage potential.

This inconvenience can be met by connecting the picture tube directly tothe anode of the video-tube and further, through a resistor, to thescreen grid. Also in this manner the anode voltage value determining thebackground brightness can be maintained constant independently of thescreen-grid voltage by means of which the contrast is controlled.

.In doing so, the impedances do not increase. However, the resistorbetween the anode and the screen grid acts as an additional load on'theanode circuit and also has an adverse effect. Consequently, thissolution is satisfactory only when existing means provide for effectivestabilisation of the black level, so that said re- 3,637,565 PatentedJune 5, 1962 sistor has to provide only a slight correction and may begiven a satisfactory value.

In television-receiving circuit arrangement-s for simultaneous contrastcontrol and background brightness control, in which an increasing firstauxiliary direct voltage yields an increase in contrast through avideo-voltage applied to the first control electrode of the picturetube, preferably its cathode, said inconveniences are avoided while,without affecting the video-circuit of the picture tube, it is possibleto obtain a desired correction, in particular to maintain the backgroundbrightness of the picture tube at a constant value, without the need foradditional stabilisation when using, according to the invention, avariable resistor device which, on operating a control member, suppliesthe first auxiliary direct voltage and an oppositely varying secondauxiliary direct voltage which is applied to a further controlelectrode, preferably to the control grid (Wehnelt-cylinder) of thepicture tube.

In its simplest form, such a circuit arrangement is made up of twomechanically coupled potentio-meters. In order to correct therelationship existing between the background brightness setting and thecontrast setting relative adjustment of the coupled part may berequired, for example for personal taste-matching. The final values ofthe potentiometers and their control ranges may be limited by seriesresistors which may be variable individually.

In order that the invention may be readily carried into eifect it willnow be described in detail with reference to the accompanying drawingwhich represents the output stage of a television receiver comprising apicture tube.

In the drawing:

FIG. 1 is a circuit diagram of a portion of a television receiver,illustrating a known system for controlling contrast and brightness witha mechanical linkage, and

FIGS. 2, 3 and 4 are circuit diagrams of a portion of the circuit ofFIG. 1 with modifications of the contrast and brightness control systemsaccording to the invention, whereby mechanical linkages are not requiredto compensate the brightness function for changes in contrast.

By means of an intermediate-frequency transformer 1 the oscillationsmodulated by the video-signal are supplied to a rectifier 2, the outputcircuit of which comprises a load resistor 3. The demodulatedvideo-voltage set up across this load resistor is applied to the controlgrid of a video-amplifying tube 4, the cathode of which is groundedthrough a resistor 5, which may be shunted. The anode of the tube '4 isconnected through an output resistor 6 to the positive terminal of thesupply. Further, the cathode of a picture tube 7 is connected to theanode.

The screen grid of the video-tube 4 is connected to the sliding contactof a potentiometer 8 which is connected to the positive terminal andnegative terminal of the supply through resistors 9 and 10 respectively.The screen grid is earthed with respect to alternating voltage through acapacitor 11.

In this circuit arrangement, which is known per se, displacement of thesliding contact of the potentiometer 8 permits the screen-grid voltageof the video-amplifying tube and consequently the working point and anyamplification of this tube to be varied so as to vary the value(amplitude) of the signal applied to the picture tube 7 and consequentlythe contrast. As is known, the term contras is to be understood to meanthe ratio existing between the brightness level, which corresponds towhite level of the signal and the brightness level corresponding toblack level of the signal.

In this type of contrast control, displacement of the working pointinvolves an undue shift also of the voltage i value associated withblack level at the anode of the videoi amplifying tube and consequentlyalso at the cathode of the picture tube so that the anode voltage dropswith an increase in screen grid voltage as is required for contrastincrease. Due to this lower voltage the drive range of the picture tubeis considerably increased so that the beam intensity increasesconsiderably. Consequently, the background brightness is noticeablyshifted in the direction of the white levels. In order to compensatethis and preferably to maintain constant the back-ground brightnesslevel, the grid voltage of the picture tube has consequently to bereduced by the same amount. In the circuit arrangement shown in FIGURE1, this is effected by connecting the control grid of the picture tube 7(for example the Wehnelt-cylinder) to the sliding contact of apotentiometer 12, the ends of which are connected throughseriesresistors 13, 14 to the supply. By suitably matching, throughseries-resistors 9, 10, 13, 14, the control ranges of the potentiometers8 and 12 it is readily obtainable that an increase in screen-gridvoltage involves a decrease in grid-voltage of the picture tube so thatits grid-cathode voltage for the background brightness remains at leastapproximately constant. By connecting resistors between the slidingcontact of at least one potentiometer and one terminal of the supply orby connecting a resistor between the two sliding contacts, variation ofthe voltage characteristics is possible so as to obtain any desiredcurvature of the characteristic. Hence, a non-linear relationship isobtainable to a degree between the controlled voltages, for example forobviating non-linearity between the screengrid voltage and the anodevoltage.

FIG. 2 shows a control device supplying the screengrid voltage u and thecontrol voltage v for the picture tube, in which only a single resistorneed be changed. This control device may consequently be substituted forthe device 8, 9, 10, 12, 13, 14 in the circuit arrangement shown inFIG. 1. Connected between the positive terminal of the supply and earthis the series-combination of a series-resistor W, a variable resistor rand a base-resistor R. The screen-grid voltage u is taken from thejunction point of the control resistor r and the series-resistor W. Anoppositely varying voltage v is obtained at the junction point of thevariable resistor r and the base-resistor R. Between the voltage a and vthere exists a linear relationship which is determined 'by the ratio ofthe resistors W and R. If the resistor r is changed between the limitingvalues 1' and r the voltages u and v change between the limiting values14 and v In this case Moreover, v should be lower than u Both values areequal if r is zero.

In the circuit arrangement shown in FIG. 2, a certain relationshipseemingly exists between the voltages u and v. This permits variation byderiving said voltages with the interposition of voltage dividers, theother end of which is connected either to the positive terminal of thesupply or to its negative terminal. tion not only of the absolute valuesof the voltages but also of their control range. In the case of earthedvoltage dividers, however, the relative difierence, that is the voltagedifference between the starting condition and the final condition withregard to the voltage in the starting condition, remains constant; theabsolute values vary in the same ratio. When considering such fixedvoltage dividers for the two auxiliary direct voltages it is found that,on calculation, the ratio p of the relative diiferences is ofimportance:

This involves variaapplied to the control grid of the picture tube, fromthe voltage v through a voltage divider connected to the positiveterminal of the supply, the proportion ratio of which was x. This isshown by broken lines in FIG. 2; its resistance value was considerablerelative to the sum of I'2+}V.

On calculation it is found that the desired voltage values areobtainable when fulfilling the condition:

where v is the desired output value, which should occur when the otherauxiliary direct voltage assumes the value al and V represents thesupply voltage. The ratio x of the voltage division is calculatedaccording to the expression The ratio between the base-resistor R andthe seriesresistor W is found from the equation:

5 1-w') V W x u (l+p)V Finally, the limiting values r and r are obtainedfrom the equation:

sion 3 are increased by the reciprocal of the voltage provoltagev'between 130 v. and 55 v. when varying the resistor r between 0 and 70kohrn; the base resistor R amounted to 2.8 kohrn, the series-resistor WWas 4 kohm. The proportion ratio of the voltage divider was 0.215. Sincewith an anode resistor 6 of 3.5 kohm the anode voltage fluctuatedbetween approximately 205 v. and 120 v. the value of the backgroundbrightness slightly increased with an increase in contrast. This isparticularly eflicient if due to incident light from the surroundingsthe background brightness has to be increased for adequately boostingthe grey stages in the proximity of black level, while on the other handthe video-voltage has to be increased for maintaining substantiallyconstant the contrast of the'reproduced picture.

FIG. 3 shows afurther form of a control device, from which the desiredoppositely varying voltages are derived.

Kohm Potentiometer 20 100 Resistor 21 5 Resistor 22 10 Resistor 23 30Resistor 24 4 It comprises a potentiometer 20, the ends of which areconnected to the positive terminal of the supply through Insofar as thevoltages u and v are loaded with a considerable current this may, ifdesired, be allowed for in known manner by increasing also the parallelcurrent of the regulating device by reducing the resistance values. Thecircuit arrangement shown in FIG. 3 shows an often desired non-linearrelationship between the voltage values u and v. The curvature of thecharacteristic may be reversed by exchanging the connections for thesupply voltage of the device 20 to 24. The characteristic may also beacted upon by inserting a resistor 25 in the lead between the slidingcontact of the potentiometer and the supply.

Finally, FIG. 4 shows a further embodiment of the circuit arrangementaccording to the invention, in which a series-resistor 30* and apotentiometer 31 are connected in series between the positive terminalof the supply and earth. The sliding contact of the potentiometer 31,from which the voltage v is taken, is earthed through a resistor 32. Thevoltage u is taken from the junction of the potentiometer 31 and theseries-resistor 30.

If the resistors 32 and 33 are equal, for example 5 kohm, while theresistor 31 has a value ten times that of the resistor 32', say 50 kohm,the voltage it varies between approximately 200 v. and 125 v., while thevoltage v rises from 0 v. to approximately 110v. Also in this circuitarrangement, a voltage divider as shown in broken lines in FIG. 2 and,if desired, further earthed voltage dividers permit, in particular, theoutput voltage v to be matched to the desired conditions.

In a circuit arrangement as shown in FIG. 4 a nonlinear relationship isobtained between the voltage values u and v similarly as in FIG. 3.

What is claimed is:

1. A television receiving circuit comprising an amplifying device havingan input electrode, a characteristic control electrode and an outputelectrode, an image reproducing device having first and second controlelectrodes, a source of video signals, means applying said video signalsto said input electrode, means connecting said output electrode to saidfirst control electrode of said image reproducing device, and means forsimultaneously controlling contrast and background brightness of animage on said image reproducing device comprising a source of a directvoltage, a resistive potentiometer circuit having first, second, thirdand fourth terminals and a single vanable resistive element, meansconnecting said first and second terminals to said source of directvoltage, means connecting said third terminal to said control electrodeof said amplifying device, and means connecting said fourth terminal tosaid second control electrode, said po tentiometer circuit comprisingresistance means connected to said variable resistive element to providepotentials at said third and fourth terminals that vary in oppositedirections with variations in said single variable resistive element.

2. A television receiving circuit comprising an amplifying device havingan input electrode, a characteristic control electrode and an outputelectrode, an image reproducing device having first and second controlelectrodes, a source of video signals, means applying said video signalsto said input electrode, means connecting said output electrode to saidfirs-t control electrode of said image reproducing device, and means forsimultaneously controlling contrast and background brightness of animage on said image reproducing device comprising a source of directvoltage, a first group of at least three serially connected resistorsconnected in series with said source of direct voltage, one of saidseries connected resistors being variable, means connecting the junctionof two of said series connected resistors to said control electrode ofsaid amplifier device, a second group of at least two series connectedresistors connected in parallel with at least two but less than thetotal number of resistors of said first group and including saidvariable resistor, and means connecting the junction of said tworesistors of said second group to said second control electrode wherebyvariation of said variable resistor varies the voltages on said controlelectrode of said amplifying device and said second control electrode inopposite directions.

3. A television circuit comprising an electron discharge device having acathode, a control grid, a screen grid, and an anode, an imagereproducing device having a cathode and a control grid, a source ofvideo signals, means applying said video signals between the controlgrid and cathode of said discharge device, means connecting said anodeto the cathode of said image reproducing device, and means forsimultaneously controlling contrast and background brightness of animage on said image reproducing device comprising a source of directvoltage, first, second and third resistors serially connected with saidsource of direct voltage, said second resistor being variable, meansconnecting the junction of said first and second resistors to saidscreen grid, fourth and fifth serially connected resistors connected inparallel with said first and second resistors, and means connecting thejunction of said fourth and fifth resistors to the control electrode ofsaid image reproducing device.

4. A television circuit comprising an electron discharge device having acathode, a control grid, a screen grid, and an anode, an imagereproducing device having a cathode and a control grid, a source ofvideo signals, means ap plying said video signals 'between the controlgrid and cathode of said discharge device, means connecting said anodeto the cathode of said image reproducing device, and means forsimultaneously controlling contrast and background brightness of animage on said image repro ducing device comprising a source of directvoltage, a series circuit of first, second and third resistors in thatorder, means connecting one end of said series circuit to said screengrid, a fourth resistor connecting said one end to one terminal of saidsource of direct volt-age, means connecting the other end of said seriescircuit to the control electrode of said image reproducing device, afifth resistor connecting said other end to said one terminal, avariable tap on said second resistor, and means connecting said tap tothe other terminal of said source of direct voltage.

5. A television circuit comprising an electron discharge device having acathode, a control grid, a screen grid, and an anode, an imagereproducing device having a cathode and a control grid, a source ofvideo signals, means applying said video signals between the controlgrid and cathode of said discharge device, means connecting said anodeto the cathode of said image reproducing device, and means forsimultaneously controlling contrast and background brightness of animage on said image reproducing device comprising a source of directvoltage, first and second resistors connected serially with said sourceof direct voltage, means connecting the junction of said first andsecond resistors to said screen grid, a variable tap on said secondresistor, means connecting said tap to said control grid of said imagereproducing device, and a third resistor connected between said tap andthe end of said second resistor connected to said source of directvoltage.

References Cited in the file of this patent UNITED STATES PATENTS2,204,992 Holmes June 18, 1940 2,470,048 Norton May 10, 1949 2,672,505Schwarz Mar. 16, 1954 2,943,145 Parker June 28, 1960

